Glass-ceramics, in particular glass-ceramics containing a solid solution of β-quartz or a solid solution of β-spodumene as main crystalline phase, have properties of major interest, in particular a thermal expansion coefficient very close to zero. These properties fully enable said glass-ceramics to be used in situations in which they are brought to relatively high temperatures, and in which they may be alternately subjected to high and low temperatures, possibly only over part of their surface. The manufacture of said glass-ceramics essentially comprises three steps, consisting of obtaining a molten precursor glass, followed by cooling and forming said molten precursor glass, then ceramming said molten, formed precursor glass. The forming of said molten precursor glass has recourse to conventional methods, such as rolling the glass between two rollers to produce sheets of constant thickness, and compressing the glass for the manufacture of items such as tableware items.
Today, the vast majority of glass-ceramic production corresponds to the manufacture of infrared or induction cooking tops, the manufacture of windows able to withstand very high temperatures such as stove windows. In most cases, flat sheets of glass-ceramic are desired, and rolling the glass proves to be a method of choice to format the sheets of precursor glass. However, some cases require the manufacture of parts of essentially constant thickness but which have a non-planar shape.
Persons skilled in the art know how to obtain glass parts of non-planar shape from sheets of glass obtained using the “Float” process for example: deforming said reheated glass sheets. Said process is routinely used and applied in particular for the manufacture of automotive windscreens and windows. Said process comprises the three following main steps:                a. heating a glass sheet up to a temperature allowing the glass to deform;        b. deforming the heated sheet (under gravity, a vacuum, molding, compression . . . ); and        c. controlled cooling of said deformed glass sheet.        
Whereas methods for manufacturing deformed or non-planar glass from planar glass sheets are well mastered by those skilled in the art, the transposition of said methods to precursor glasses of glass-ceramics is problematic. The range of temperatures under which the deformation of the precursor glass is possible generally covers the range of temperatures at which ceramming occurs, or at least at which said ceramming is initiated.
A few methods have been developed taking these constraints into account, in particular the method described in patent application FR 2 735 562. Said method proposes to directly form, from a delivered mass of a molten glass precursor of the final glass-ceramic (a glass gob), a non-planar precursor glass by molding which after ceramming, is converted into a glass-ceramic. This method is very efficient for the mass production of said non-planar glass-ceramics, but requires a complex, costly installation whose use is not economically advantageous for the production of prototypes or the production of limited series.
The methods described in patent applications FR 2 726 350 and FR 2 777 559 form alternatives, based on one same principle, to produce non-planar glass-ceramics by deformation. Application FR 2 726 350 describes a method to produce cooking tops having folded edges, and application FR 2 777 559 a method for producing cooking tops whose openings have a deformed surround. These two methods include very rapid heating of a local area of the sheet of precursor glass, followed by forced deformation of said heated local area. These methods carried out with burners (which imply a significant portion of heat transferred by forced convection) are only suitable to generate local deformations.
Patent applications DE 100 47 576 and EP 1 171 391 describe another type of method enabling the manufacture of non-planar glass-ceramics, based on the use of infrared energy (very high energy process). The infrared energy can heat an entire sheet of precursor glass rapidly before it is deformed by different means. Said method requires the use of infrared radiating equipment having high energy densities and sophisticated tooling (moulds) surfaces. The cost of the equipment is high and the operation requires special precautions, in particular impeccable cleanliness to avoid damaging the highly transparent surfaces or highly reflective surfaces used in infrared lamp technology.
Patent applications DE 101 02 576 and FR 2 866 642 teach the forming of sheets of precursor glass during the ceramming cycle. In particular, application FR 2 866 642 describes that the precursor glass can be deformed during a short period corresponding to the time at which a temporary decrease in viscosity is observed due to latent crystallization heat. The methods proposed by these documents DE 101 02 576 and FR 2 866 642 make it possible to manufacture parts of complex shape, but they entail the use of a complex installation to conduct ceramming of the sheets of precursor glass. Insofar as the ceramming cycles last one hour, even more, and take place in continuous production furnaces, persons skilled in the art will understand that efficient production requires extensive tooling and corresponding major investment. For example, as described in application FR 2 866 642 a ceramming gallery having specific characteristics to conduct forced deformation requires a heavy investment.
U.S. Pat. Nos. 3,681,043 and 4,009,064 describe methods in which the steps of forming and nucleation/ceramming are not clearly separated. The annealing or further heat treatment mentioned in U.S. Pat. No. 4,009,064 is carried out on a formed article at least partially cerammed.
In said context, the present invention proposes a method for manufacturing non-planar (non flat) products in glass-ceramic. Said method is particularly suitable, in particular from an economic viewpoint, for the production of limited series or of small quantities of non-planar products in glass-ceramic. It has the advantage that it can be implemented with an installation requiring moderate investment.